While green hydrogen is an important element in the net zero plans, current and future salt caverns will be insufficient to meet all our hydrogen storage needs. Additional new underground solutions will be needed, says Martin Wright, founder and executive chairman of Gravitricity.
Green hydrogen will become an important part of the low-carbon energy systems of the future.
While the debate is still open about the role this CO2-neutral gas will play in domestic environments, it is clear that green hydrogen – produced via the electrolysis of water using renewable energy – will play an important role in industry, and as a strategic power reserve.
Hydrogen is already an important part of a range of industrial processes, from the production of ammonia for use in fertilizers to refining and countless other applications – but in the vast majority of cases we still rely on what is known as ‘grey’ hydrogen which is produced through the highly polluting steam reforming process using methane or natural gas.
Global hydrogen production is currently around 120 million tonnes per year, making its CO2 emissions greater than those of aviation, and almost greater than those of aviation and shipping combined.
If we are to have any hope of reaching net zero, our first step should be to replace this high-carbon feedstock with a low-carbon alternative.
Industrial users
At the same time, we need to look at using green hydro as a high-temperature fuel – for things that can’t be electrified or decarbonized in other ways – and again, many of these will be industrial processes, such as green steel production.
Furthermore, green hydrogen can play a strategic role in long-term energy storage – absorbing spare electrons when the electricity grid is overcapacity, and generating green power when renewables cannot. This could be from day to day, or for weeks, or even longer.
While the full extent of green hydrogen’s role is yet to be determined, it is clear that demand will be high. The respected Commission on Climate Change’s 6th Carbon Budget shows that 250 to 460 TWh of hydrogen could be needed by 2050, which would represent 20 to 35 percent of the UK’s final energy consumption.
But if this is the case, we will have to find ways to store it safely and affordably.
With natural gas – we can rely on the enormous natural reserves under the sea, and on ‘line pack’ – gas is actually stored within our existing pipeline network.
But hydrogen is different. Green hydrogen is a wonder gas: it can be made from water and renewable energy and produces no emissions (when used in fuel cells). But on the other hand, it is highly flammable, with much smaller molecules than the natural gas we depend on, and it cannot be stored in the same way without significant changes to the current network.
But as production of green hydrogen increases, the ability to store large quantities of this gas for extended periods will become a crucial building block in our low-carbon future.
Salt caves
Currently, the only proven technology for storing large quantities of pure hydrogen is in underground salt caverns, in areas with suitable geology.
Britain currently has 0.025 TWh of hydrogen storage in the salt caverns, with two notable projects under development, namely HyKeuper in the North West of England and Aldbrough near the strategically important Humber, which will provide a further 1.3 TWh and 0.5 TWh of storage capacity respectively add.
But it will take many years to build, and while Britain is fortunate to have suitable geology for salt cave storage, there simply isn’t enough of it. Furthermore, they can only be built in very specific locations, which do not always match current or future storage needs.
A recent report from global energy consultants Arup in collaboration with the University of Edinburgh and supported by the British Geological Survey suggests that we may have overestimated the capacity of the UK’s salt caves by as much as 90 percent and states: “Without immediate action, limits to hydrogen storage capacity are at risk Britain is going to net zero.”
Medium storage
Therefore, the future large-scale application of green hydrogen in industrial processes and for heat, transport and backup generation will also require safe and cost-effective medium-sized hydrogen buffer storage – up to 100 tonnes per unit – that can be located where necessary, both in distribution networks and on the point of use.
This could apply to industrial customers – such as ammonia or steel producers – where there may be a need for 500-1,000 tonnes of hydrogen storage on site to provide buffer storage of one to two days.
In addition, as parts of the UK gas network are repurposed for hydrogen, there will be demands for localized ‘linepack’ storage at nodes – in the hundreds of tonnes, coupled with the use of large-scale salt cavern storage.
In other words, it’s not “either/or” for large-scale and medium-sized storage; we will need both.
National Gas – the owner and operator of the UK gas transmission system – is already exploring the potential of hydrogen transport using the current network infrastructure, with the need for medium-sized storage central.
This initiative, known as Project Union, is a ground-breaking project to create a UK hydrogen backbone, capable of transporting 100 percent hydrogen and connecting hydrogen production and storage to industrial end-users across Britain.
Through the phased repurposing of the existing high-pressure gas transmission network infrastructure – in addition to the construction of selected new pipelines – Project Union aims to create a hydrogen network of up to 2,000 km, equivalent to 25 percent of the current UK methane transmission network.
As part of this, medium-sized ‘buffer storage’ of hydrogen will be crucial.
The idea of buffer storage is not new. In the last century, in the era of manufactured ‘city gas’, buffer storage came in the form of the gigantic, drum-shaped gas holders that still exist in so many of our cities.
But in the modern age, above-ground storage is expensive and will come with planning challenges – and could pose a potential security risk at a time when bad actors are actively targeting energy infrastructure.
This means that there are no clear and obvious ways for end users and for network operators to store significant quantities of green hydrogen safely and affordably as part of the new low-carbon economy.
The race to net zero is a major challenge for all countries and there is no ‘one size fits all’ technology that can get us there.
Green hydrogen will certainly have a role to play – but to unlock its potential we need to find new ways to store this flammable ‘wonder gas’. Salt caverns will provide large storage volumes, but we will also need medium-sized storage that can be located at multiple nodes around the future gas network and also in close proximity to large industrial customers. I believe the answer to this is right under our feet.